We present direct evidence that the giant magnetocaloric effect recently discovered in the ${\mathrm{Gd}}_5{(\mathrm{S}\mathrm{i}}_{1.8}{\mathrm{Ge}}_{2.2})$ alloy is associated with a field-induced first-order structural transition from a ${P112\mathrm{}}_1/a$ monoclinic (paramagnetic) to a Pnma orthorhombic (ferromagnetic) structure. A large volume contraction of $\Delta V/V\cong 0.4\%$ takes place spontaneously at the transition temperature, $T_C\cong 240\mathrm{K}.$ The reported structural transition can be induced reversibly by application of an external magnetic field, producing strong magnetoelastic effects.

• Received 17 September 1998

DOI:https://doi.org/10.1103/PhysRevB.58.R14721